Hypotheses being tested during exercise using stable isotope tracers are: 1) lipolysis is influenced by body composition and 2) is stimulated by beta adrenergic activity; 3) changes in glucose and/or insulin concentration are responsible for the relationship between lipolysis and fatty acid release; 4) adenosine inhibits peripheral lipolysis in high intensity exercise, 5) fat oxidation is limited by availability, and 6) long chain fatty acid oxidation is limited by transport into the mitochondria via carnitine acyltransferase.